Silicon-based solar cells, or photovoltaic cells, require several processing steps in order to be able to convert incident light into current. One of these steps involves the generation of an emitter, which is most commonly accomplished by the thermal drive-in of phosphorous into a boron-doped silicon wafer. This process results in the generation of a so-called dead layer, which gives high recombination rates of the generated charges and is detrimental to the efficiency and power density level of the solar cell. Additionally, this process produces a so-called phosphosilicate glass (PSG) layer on top of the wafer, which contains phosphorous, silicon and oxygen and this PSG layer has to be removed in order to be able to proceed in cell manufacture. After the thermal drive-in process, the phosphorous depth profile shows a plateau of high-concentration extending from the surface to several tens or hundreds of nanometers deep, depending on process conditions. Ideally, the concentration near the surface would be high (i.e. 1020-21 atoms/cm3) in order to be able to contact the electrodes well.
A principal goal of multi crystalline photovoltaic cell manufacturers is to reduce the cost of the energy delivered by their solar cells. This can generally be accomplished in one of two ways, either reduction in overall cell manufacturing costs and/or improvement in solar cell conversion efficiency. In an effort to achieve the latter objective current manufacturing processes apply a post-emitter etch after the phosphorous diffusion, which removes the PSG layer by dipping the wafer in HF. Previous experiments have shown that an additional treatment after the HF-dip can result in higher cell efficiencies, up to 0.3% absolute. Currently, a product of Mallinckrodt Baker, Inc., namely product PV-160, is used in this additional step. However, use of this product generally requires processing of the wafer substrate in a heated bath (70° C. or higher) of the product.
It is highly desirable that compositions be available that are capable of producing higher power density in solar cells in equal or lesser processing times and at reduced temperatures by improved etching of remnants of the PSG layer as well as deeper etching of the dead layer, compared with results obtained with the currently used PV-160 product.